1. Field of the Invention
The present invention relates to a developing device for use in an electrophotographic image forming apparatus such as copying machine and laser beam printer, and more particularly, to a developing device wherein a two-component developer is used.
2. Description of Related Art
In this kind of developing device, it is generally arranged to make the toner density of the developer, that is, the amount of toner to carrier, uniform and to have the toner to be frictionally electrified properly. It is further arranged to supply a part of the toner for a developing process at a midway point in a path of circulation in which the developer is stirred and transported.
On the other hand, it is preferred that the density of the visual image after development be maintained constant in relation to a specified density set. However, the density of toner is lowered every time when the developer is used for development thereby causing the density of the image to be lowered.
Japanese Patent Publication TOKKAI SHO 62-28780 discloses a developing apparatus which solves the problem. The apparatus is arranged to detect toner density or visual image density after development by a density detecting means and replenish toner to the developer being circulated and transported when the detected density is below a predetermined value, which has heretofore been practised.
However, when toner is replenished in accordance with the shortage of toner density, it results in uneven toner density and shortage of electrification if the toner is not stirred sufficiently and causes irregularity of image and shortage in image density. It is, therefore, considered to continue transporting developer even after the developing process is finished. However, it may cause to worsening of the quality of developer by excessive stirring if the developing apparatus is simply kept on driving. Besides, it consumes considerable electricity and causes high noise even when the developing process is suspended.
When a color image is formed, the density of the original, that is, the ratio between black and white on the surface of the original, is as high as 50-80% compared to that of an ordinary image which is below 25% and considerable amount of toner is consumed. Accordingly, the amount of toner to be replenished to the developer being stirred and transported is necessarily increased. However, if a large amount of toner is replenished, the toner can not be sufficiently stirred before it reaches the position of development and causes irregularity in toner density and an uneven image since the distance from a toner replenishing inlet to the position of development in a path of circulation is limited in practical design.
Japanese Patent Publication TOKKAI SHO 62-8176 discloses a developing apparatus which deals with the problem described above. The apparatus is arranged to vary transporting speed distribution by providing notch portion on a transporting lead member in a stirring and transporting means. Such arrangement raises stirring efficiency since the flow of developer is varied with variation of transport speed in the transporting direction.
However, the variation in the speed of transportation of developer in the transporting direction can only provide slow and fast movement partially in the flow of the developer, and sufficient stirring for repeatedly and positively replacing developer over all portions can not be achieved. In the case when a large amount of toner is replenished for color image development, the developer which has received the toner can not make the density sufficiently uniform, and further improvement is required. Furthermore, in detecting the toner density by a sensor, if the amount of developer in the density detecting portion is not secured at a predetermined value, the density detected differs from actual value and there arises a scatter in the value.
U.S. Pat. No. 4,711,551 discloses an apparatus which deals with the problems described above. The apparatus is arranged to store developer in a density detecting portion by providing an interrupted portion opposite to the density detecting portion.
However, it is necessary to successively renew the developer stored therein in order to be able to continuously detect toner density in each portion since the developer is stored in the density detecting portion without having been stirred or transported. There is only one way to surely push forward the stored developer by the developer following. In order to surely push forward the developer, it is necessary to transport all the developer with transporting force in all areas of circulation when the developer is transported, however, it causes the stirring efficiency to be lowered and tends to cause irregularity in toner density and insufficient electrification.